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Olympus Mons
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Reged: 02/12/12

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Mirror shifting, thermal change or them both?
      #5560756 - 12/08/12 11:41 AM Attachment (30 downloads)

Hi all!
As you read from the title after one year owning my C8 I finally had to face one of the worst malfunctioning of this telescope: the mirror shifting.
Last week I had a very good seeing and I decided to mount my setup in order to photograph NGC 891. All seemed to go well until 8.30 pm (I begun 6.30 pm) when I had to stop the sequence of shots: phd lost my star guide that had started to become fainter as the minutes passed. I thought soon it was due to a thin dew film on the corrector plate but I was wrong: I looked at the plate but it was dry. I post this comparison between the frist shot and the last one. I have to notice that I began the shots when the telescope was in temperature balance since one hour even though is possible the effect is due to a thermal shifting. I also focused on a star next to NGC 891 in order to avoid the mirror flip, but it seems the problem was neither there.
Meanwhile shooting, the telescope was gradually moving toward zenith, tracking the object. At the final shot the scope was vertical. Do you think the problem could be due to this change on the telescope's orientation? Maybe the primary mirror that was moving very slowly, has gradually increased pressure on the focus screw as the telescope moved to the zenith...just an hypotesis . How can I correct this problem?

Thank all and clear skies


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rigel123
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5560781 - 12/08/12 12:01 PM

How long were your exposures? It looks like you either have drift in your tracking or your scope may be a bit out of collimation. It's also possible that your focus has changed due to changing temperature. I am assuming your image is from the center of your FOV.

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Olympus Mons
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Reged: 02/12/12

Loc: Italy
Re: Mirror shifting, thermal change or them both? new [Re: rigel123]
      #5560789 - 12/08/12 12:12 PM

Thanks rigel23 for your fast reply!
I took 12 exposures by 10 minutes each. The images presented above are both crop of the center of the FOV.
It wasn't an easy night with the guiding, there was an annoying wind disturbing the job of my Magzero Mz-5m on OAG. It think this the most important reason of the drift on the stars because collimation is ok for sure , since I checked it out on Vega before the beginnig of the exposures.

Clear Skies and thanks again


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Footbag
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5560811 - 12/08/12 12:27 PM

As Warren said, it looks like you focus shifted with the temperature slightly, but there is something else going on. The stars look like they have slight drift, but if it was windy maybe the guider was just fighting the wind. Has this happened before?

As far as mirror shifting, run the focus all the way in and out a few times to re-distribute the grease. Also try to come to focus in a counter-clockwise direction.

Edited by Footbag (12/08/12 12:29 PM)


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Peter in Reno
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5560814 - 12/08/12 12:28 PM

If you are using OAG, then it should take care of mirror flop. If you blow up the image, do the stars look like donuts especially at dimmer stars? If so, then the scope went out of focus. It has happened to me. I usually check/refocus every hour during the first two or three hours and after that the focus should stabilize. When you lose focus, the stars get dimmer like your guide star. Or it's possible that thin and high clouds may have came by.

The focus can vary from night to night. It could be an unexpected significant change in temperature or even significant sudden mirror flop that OAG could not compensate. Sometimes I can image all night without changing the focus.

You also said there some wind which could easily be a factor.

So better safe than sorry is to check/refocus at beginning of imaging session.

Peter


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Eddgie
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5560854 - 12/08/12 01:01 PM

The stars in the picture on the right seem to show coma.

This coma could be there because the scope may not be in proper collimation.

If you did not approach final focus using CCW motions of the focuser (if you overshoot, back off and again very slowly tweak to final focus using CCW motion), as the mirror settles against the grease film on the baffle, the focus and the collimation will change.

This means you should go back to step one.

Re-collimate using a star high in the sky. After each move of the secondary, reposition the star exactly in the center of the field using counter-clockwise turns of the focuser knob. Always always approach final focus using CCW motions only. Don't rock back and forth.

Collimating this way "Lifts" the focuser knob side of the mirror so that it always comes to rest with the same tilt against the baffle and with pressure being held against the threads on the focuser rod.

Don't worry if you very slightly over-shoot best focus. In fact, because of the field curvature of the scope, the best result for all of the stars in the frame will be when the very center is very slighty out of focus, but only when you approach final focus using counter-clockwise motion.

The reason is that the field of the scope is curved at the focal plane with the center being closer to you than the outside edge.

If you focus at the exact center, the stars at the outside will be quite defocused.

If you focus at the outside, the stars at the center will be out of focus quite a bit.

But if you focus a bit away from the center, stars at the center will still appear pretty sharp in the image, and the defocus blur at the outside of the field will be reduced.

So bottom line is simply... Re-collimate, and always always always approach final focus using only CCW motion. If you over-shoot a tiny bit when either imaging or when viewing visually, don't back up. For images, stars at the outside will be sharper at only a very tiny expense of sharpness at the center of the field, and visually, your eye can automiatically re-focus the center of the field if you overshoot using CCW motion.


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Olympus Mons
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Reged: 02/12/12

Loc: Italy
Re: Mirror shifting, thermal change or them both? new [Re: Eddgie]
      #5560888 - 12/08/12 01:25 PM

Thanks all for the advice!
@footbag. Perhaps I wrongly judged the problem of the wind. It was a bit disturbing but the graph given by phd was enough flat; there were few oscillation at the beginning but changing few setting it begun more sharp so the wind maybe wasn't the primary reason of the drift. I think maybe an important factor could be the coma who affects a lot SC in general, my own one too. I had to fight with it also other times, but when the atmosphere was steady the problem was not so evident and annoying.

@peter. What you mean with "blow up on the image"?. Sorry but I don't know this technical expression!!

@Edgie. Do you think I have to exchange the stock focuser of my telescope with one of higher quality such as the starlight's one for have more sharp movement on focus? Or could be best invest money on a coma corrector such as the Alan Gee? It would also enhance the number of object I can photograph thanks to the reduced focal...

Thanks all again and clear skies


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Peter in Reno
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5560912 - 12/08/12 01:42 PM

I meant zoom the image to look closely at the stars to see if they got de-focused.

I don't think changing the focuser will help reduce mirror flop. It may help reduce mirror shift while focusing.

Peter


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GlennLeDrew
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Re: Mirror shifting, thermal change or them both? new [Re: Peter in Reno]
      #5560934 - 12/08/12 01:58 PM

Focus has definitely changed over that time period. With the amification factor of an SCT's secondary, small shrinkage in tube length with cooling quickly moves the focal surface farther back. You may need to check focus as often as about every 20-40 minutes, more often earlier on, usually.

If these images are from near the center of the frame, collimation is definitely off.

Assuming your mount has dual axis tracking, using an off-axis guiding unit will (should) obviate mirror shift as a positional error, keeping your framing constant. But mirror flop willl almost certainly introduce a change in focus, too. And if the mirror flop is large, collimation can be somewhat affected.


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Peter in Reno
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Re: Mirror shifting, thermal change or them both? new [Re: GlennLeDrew]
      #5561621 - 12/08/12 10:41 PM

I agree that your scope is likely to be out of collimation. If the galaxy is at the center of the image, then the out of focused stars would show a black hole in the center of each stars but your image showed black holes offset of each star.

I think your issue is twofolds. One is collimation and other is need to refocus periodically early during image session.

Peter


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Olympus Mons
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Reged: 02/12/12

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Re: Mirror shifting, thermal change or them both? new [Re: Peter in Reno]
      #5564416 - 12/10/12 04:03 PM Attachment (11 downloads)

Thanks all for advice and sorry for delay of the answer!!

@ Peter in Reno. I have bob's knobs and I hadn't problem of collimation before. I took images of Jupiter few weeks ago and they seemed very sharp. Since then the telescope wasn't used at all. A question on the your previous answer: why should the OAG compensate the mirror flop?
Another data on the collimation: yesterday I attempted again to take few shots on NGC 891 with bad results again(I couldn't find good guide star...just stars that put the galaxy on the border!! Ahhh it is going to be an odyssey for take this galaxy !!) and this image posted is the result: a single shot of 9 minutes, no hints of problems on collimation, except the usual coma on borders. I haven't modified the collimation and for this reason I think this test eliminates, or at least reduce, the possibility the problem was the collimation on last session. What do you think about it?


@GlennLEDrew. Can you suggest me a permanent solution to the mirror flop? I read on few pages on the Web I could lock my primary mirror and focus with an external focuser. Do you agree with this opinion?



Thanks all again and Clear skies


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Peter in Reno
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5564509 - 12/10/12 04:58 PM

OAG helps deal with mirror flop flexure because both guide and main cameras share the same imaging train and therefore both cameras are in sync. If the primary mirror tilted or flopped a little bit during autoguiding, the guide camera will know about it. But if using the guide scope, the guide scope will have no idea that the mirror flopped during tracking.

When it comes to imaging with SCTs, there are several variables to deal with. Mirror flop is the most common. It's the way all SCTs with moving primary mirrors are designed. It does not mean the design is flawed. You just have to gain experience and learn to deal with as I have.

For example, if you forgot to finish focusing in CCW direction, then you can expect the unexpected like the primary mirror slip far enough to affect the focus, star elongation or both. If you finish focusing in CCW, you may have a better chance of getting good images.

If you are patient enough, you can check/refocus periodically to make sure the focus is intact during the whole imaging session. Most of the time I have good luck and the focus can remain intact all night without refocusing. When I image an DSO that could track near Zenith, I notice the focus would be more affected because the heavy mirror may have a better chance to shift enough due to gravity and can affect focus.

I am not sure the difference between your latest image and original image. The original image clearly showed scope out of collimation because the "donut" skewed to the left of the stars when out of focused when the "donut" should have been at the center of the stars. Is your latest image pointing to Zenith long after you finished focusing or right after you finished focusing? I cannot give any other explanation of what happened in your original image. It's possible the mirror slipped, tilted and got stuck at an undesired position that caused the original image appear to be out of collimation when the scope was pointing toward Zenith.

I would not look into locking the mirror. It will give you headaches and may be too expensive. I don't think there are kits available to be able to lock the mirror when you want to. You may be better off to replace your C-8 with C-8 EdgeHD like I did. I find the EdgeHD a significant improvement over regular SCT not because of locking the mirror but also flat field across the FOV of the image.

Peter


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GlennLeDrew
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Re: Mirror shifting, thermal change or them both? new [Re: Peter in Reno]
      #5566243 - 12/11/12 04:31 PM

If there is an inexpensive way to fit a mirror lock, it can prove beneficial. However, be aware that adding an external focuser will introduce a bit of an increase in system focal length and a concomitant increase in the f/ratio.

If you decide to stick with the focusing primary, it might pay to do some experimentation with the focusing action...

As the scope shrinks during cooling, the primary and secondary mirrors are moved closer together, thus moving the focus farther back. To compensate, the primary must be moved rearward, which is in the direction that gravity naturally tugs on it.

Now, the question is this. Is the grease on the baffle viscous enough to *potentially* hold the mirror in place by itself, particularly when the scope is closer to horizontal than vertical? If so, then the focusing action should first pull the primary well back (overshooting), and then gradually back inward. This will ensure that there is no slack on the screw feed over which the mirror can slump back downward.

All this fiddling is (or likely) required because of backlash and other 'looseness' in the focusing mechanism.

Your travails with finding a handy guide star is exactly why I jettisoned the OAG solution to guiding decades ago. May I instead suggest something like the Orion Star Shoot Autoguider, with lightweight 50mm sco


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GlennLeDrew
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Re: Mirror shifting, thermal change or them both? new [Re: Peter in Reno]
      #5566246 - 12/11/12 04:33 PM

If there is an inexpensive way to fit a mirror lock, it can prove beneficial. However, be aware that adding an external focuser will introduce a bit of an increase in system focal length and a concomitant increase in the f/ratio.

If you decide to stick with the focusing primary, it might pay to do some experimentation with the focusing action...

As the scope shrinks during cooling, the primary and secondary mirrors are moved closer together, thus moving the focus farther back. To compensate, the primary must be moved rearward, which is in the direction that gravity naturally tugs on it.

Now, the question is this. Is the grease on the baffle viscous enough to *potentially* hold the mirror in place by itself, particularly when the scope is closer to horizontal than vertical? If so, then the focusing action should first pull the primary well back (overshooting), and then gradually back inward. This will ensure that there is no slack on the screw feed over which the mirror can slump back downward.

All this fiddling is (or likely) required because of backlash and other 'looseness' in the focusing mechanism.

Your travails with finding a handy guide star is exactly why I jettisoned the OAG solution to guiding decades ago. May I instead suggest something like the Orion Star Shoot Autoguider, with lightweight 50mm scope. The roughly 3 degree field ensures a suitable guide star every time, and the very low mass is hardly expected to introduce flexure. A couple of friend's use this package on 11" and 14" SCTs (both working at f/7) with good results.


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Olympus Mons
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Reged: 02/12/12

Loc: Italy
Re: Mirror shifting, thermal change or them both? new [Re: GlennLeDrew]
      #5572626 - 12/15/12 01:31 PM Attachment (8 downloads)

I've tried days ago the techinques you proposed me about focusing CCW. I checked focus periodically and finally I obtained fine images: it worked very well! I post you the final shot of a sequence of three subsequent photograph of NGC 891. I managed to focus CCW on few shot before and then I've been able to mantain focus steady, having a good roundness of the stars for three shots. Is this evident also to you?

Thank you very much again for having helped me to solve this problem!

Clear skies


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Peter in Reno
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Re: Mirror shifting, thermal change or them both? new [Re: Olympus Mons]
      #5572678 - 12/15/12 02:18 PM

Excellent!!!!

I looked up your autoguider, Magzero Mz-5m, and it appears to use same or similar CCD as Orion SSAG. I don't think it's a sensitive camera to work with OAG. It uses less sensitive CMOS sensor instead of CCD. It's naturally difficult to find suitable guide star with OAG due to narrow FOV. I used to have Orion SSAG and I replaced it with a much more sensitive autoguider called Lodestar. It was day and night. I never had to hunt for guide stars with my C-8 and OAG. Several times 4 guide stars would show up after framing a DSO with main camera. Lodestar costs a bit more but the rewards are much greater.

Congrats again getting your issue resolved. I know SCT scopes are not easy to deal with but once you understand what you need to deal with SCT, it gets much easier.

Peter


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